1. Field of the Invention
The disclosures discussed herein relate to a projection optical system and an image display apparatus having the projection optical system capable of enlarging an image to display the enlarged image on a screen.
2. Description of the Related Art
There is an image display apparatus having a projection optical system known in the art, which is capable of being placed at a relatively closer position to a screen than a position at which the related art image display apparatus is allowed to be placed. Such an image display apparatus is called a “close range projector”. The purposes of the close range projector being devised are as follows. First, the projector will not project light too bright to blind a presenter or a demonstrator who stands close to the screen, and secondly, the projector will not emit an exhaust gas or noise to adversely affect the audience who watches and listens to the presentation.
Such a close range projector may utilize a related-art projection optical system (coaxial and rotationally symmetric) to reduce a distance between the projector and a screen surface by widening the angle of view of the projection optical system, or may utilize a curved mirror. Thus, the close range projector having the related-art projection optical system capable of widening the angle of view may achieve the aforementioned purposes by improving the related-art technology. However, in the above related-art projection optical system, an outer diameter of a lens arranged close to the screen side may need to be increased, which may result in an increase in the size of the projector itself. By contrast, the close range projector having the curved mirror may be capable of projecting light at an extremely close range without increasing the size of the projector itself.
Examples of the close range projector utilizing the curved mirror are disclosed in Japanese Patent No. 4329863 (hereinafter called “Patent Document 1”) and Japanese Patent No. 3727543 (hereinafter called “Patent Document 2”). In the close range projector disclosed in Patent Document 1, a concave mirror is arranged behind the lens optical system for projecting light. In the close range projector disclosed in Patent Document 2, a convex mirror is arranged behind the lens optical system for projecting light. In either cases, arrangement accuracy between the components may be improved by simply arranging lenses and a mirror sequentially. However, in both cases, a long distance may be required between the lens optical system and the mirror, which may result in an increase in the size of the projection optical system.
Meanwhile, examples of the close range projector capable of reducing a distance between the lens optical system and the mirror are disclosed in Japanese Laid-open Patent Publication No. 2009-157223 (hereinafter called “Patent Document 3”) and Japanese Laid-open Patent Publication No. 2009-145672 (hereinafter called “Patent Document 4”). These close range projectors disclosed in Patent Document 3 and Patent Document 4 include a reflector configured to bend or fold an optical path having a long distance between the lens optical system and the reflector, which may reduce the size of the projection optical system.
In the projector disclosed in Patent Document 3, the size of the projection optical system may be reduced by sequentially arranging a concave mirror and a convex mirror and a convex mirror subsequent to the lens optical system. In the projector disclosed in Patent Document 4, the size of the projection optical system may be reduced by arranging a plane mirror behind a concave mirror.
However, either of the projection optical systems disclosed in Patent Documents 3 and 4 have a long distance between an image display device and a curved mirror. Hence, if a user desires to place the projector main body further at a position closer to the screen than allowable positions at which the related art projectors are placed, a length of the projection optical system itself may become an obstacle.
Japanese Patent No. 4210314 (hereinafter called “Patent Document 5”) discloses a technology for resolving such a limitation of “the size of the projection optical system itself”. More specifically, Patent Document 5 discloses a projection optical system having an image display device, a display surface of which is orthogonal to a screen surface. With this vertical configuration, the projector main body may be capable of being placed even closer to the screen than the allowable positions of the related art projectors because the obstacle due to the length of the projection optical system itself may be resolved or eliminated.
However, although the projection optical system has the vertical configuration such as the one disclosed in Patent Document 5, which is capable of projecting an image at an extremely close range while reducing its size, divergence of light incident upon a mirror system from the lens optical system may need to be increased in order for the projector placed closer to the screen to display a larger image on the screen projection. However, if divergence of light is increased, the following three problems may occur.
That is, first, it may become difficult to correct aberration of projection luminous flux passing through parts other than an optical axis of the lens system. Second, the diffused luminous flux strikes a lens surface closest to the first mirror before striking the concave mirror. Third, the light reflected off the concave mirror strikes the first mirror in the middle of a route toward the screen before reaching the screen.
As illustrated in the technology disclosed in Patent Document 5, if a lens surface closest to the first mirror is concave, each of lens surfaces is gradually protruded toward the first mirror side with the increasing distance from an optical axis to the position of a corresponding lens. Further, in view of aberration correction and error sensitivity, when a lens surface closest to the mirror is concave, the refracting angle of the beam is extremely large in the projection optical system that manages the luminous flux having increased divergence. Hence, it may become difficult to correct aberration in an entire region of the screen. In this case, since the refracting angle of the beam is large, image quality may drastically deteriorate even if the slightest shifts are present in the arrangement of the components of the projection optical system.
Further, in the vertical projection optical system disclosed in Patent Document 5 having a projector lens arranged parallel to the screen, particles such as dust may be easily be attached to the projector lens or mirrors compared to the projector lens in a horizontal projection optical system having a projection optical system arranged orthogonal to the screen. In addition, in the configuration of the above case, attached dust may fall onto the vertical projection optical system. Accordingly, if particles or dust attached to the projection system remain attached, the attached particles may be displayed on the screen without automatically coming off from the projector lens due to the gravitational effect.
Moreover, since the angle of view of the lens optical system is narrow in the projection optical system disclosed in Patent Document 5, the particles attached to one of the two mirrors, each of which causes the projection luminous flux to reflect off its surface, may affect the amount of light projected onto a projection surface of the screen. In this case, even if a dustproof glass is arranged between the mirror and the projection surface of the screen on which the projection light is projected, it may be difficult to prevent fine particles or dust having a size of 0.01 mm or less from intruding onto the projection surface.